The long-term objective of these studies is to elucidate the factors which regulate the biosynthesis of polyunsaturated fatty acids. The fatty acids of the linoleate, oleate and palmitoleate biosynthetic pathways have been prepared in a radioactive form by total organic synthesis. These acids have been used in enzyme experiments to determine to what extent rates of desaturation and chain elongation can be used to predict the type of acid which is produced and thus made available for incorporation into lipids. In other studies we have prepared seven isomeric methyl branched isomers of eicosa-8,11,14- trienoic acid to determine how structural modification alters desaturation at the 5-position. Studies presently being carried out include the following: 1) Appropriate intermediates normally produced during the microsomal chain elongation of fatty acids have been prepared by total organic synthesis and are being used to study various aspects of the chain elongation reaction. 2) The action of various inhibitors on the various individual steps in the chain elongation of fatty acids are being investigated. 3) Arrhenius plots of the individual reactions in the chain elongation reaction are being determined.